HYAKUTAKE X-RAYS SHOW ABILITY TO MONITOR COMETS AND SOLAR WIND

A supercomputer simulation of Comet Hyakutake's interaction
with the solar wind demonstrates that resulting X-ray emissions
can be used to monitor comets and solar wind phenomena, NASA-
funded researchers write in today's issue of "Science."

The simulation was conducted using an Earth sciences
supercomputer at the NASA Goddard Space Flight Center,
Greenbelt, MD. The results match and explain March 27, 1996,
observations of Comet Hyakutake by Germany's ROSAT satellite,
the first detection of X-ray emissions from any comet. The
model also supports a leading theory for how the X-rays are
generated.

"Cometary X-rays present a potentially powerful new tool to
monitor comet activity far from Earth, as well as the
composition and flux of the solar wind," said co-author
Dr. Tamas Gombosi of the University of Michigan, Ann Arbor. "By
capturing these X-rays' detailed energy spectrum, it might be
possible to monitor the propagation and evolution of
spectacular solar wind phenomena, such as the coronal mass
ejections seen this January and April."

About one percent of the solar wind, which flows from the
Sun out past Pluto, is composed of minor ions: atoms (such as
oxygen, carbon and neon) that have been nearly stripped of
their electrons and thus have a high positive charge. Dr.
Thomas Cravens of the University of Kansas theorizes that these
minor ions steal electrons from neutral atoms and molecules of
cometary origin. The electrons are first seized in excited
states, traveling in the ions' outer orbitals. As the electrons
fall to lower orbitals, Cravens' theory asserts that X-rays are
emitted, in addition to other forms of radiation.

"Considering the magnitude and shape of the emission, we
believe the most satisfactory theory to be this mechanism of
charge exchange excitation," Gombosi said. "Other explanations
produce neither the crescent pattern nor the intensity observed
by ROSAT and duplicated by our simulation."

Within this pattern, some electron orbital transitions emit distinct
wavelengths of X-rays that can be measured. The computer simulation shows
that the overall X-ray spectrum for Comet Hyakutake depends mainly
on the solar wind composition, and not on the comet. Because
of this independence, researchers can determine the relative
size of the comet's atmosphere from the proximity of the
brightest X-rays to the icy nucleus.

"In Hyakutake, the brightest X-ray region was 18,700 miles
(30,000 kilometers) ahead of the comet, on the Sun side," said
University of Michigan co-author Dr. Michael Combi. "If the comet
has enough of an atmosphere, the solar wind minor ions recombine
with electrons far from the nucleus. If the comet were producing
less atmospheric gas, the place of maximum emission would be closer
to the nucleus," Combi said.

This theory will be tested on Comet Hale-Bopp, which is
scheduled to be observed by Japan's ASCA X-ray satellite this
September. "Comet Hale-Bopp should have the emission shifted
further sunward; it is bigger than Hyakutake," Combi said.

Active comets are typically first observed in visible light
at large distances from the Sun. After discovery, the orbits
of comets can be established with very high accuracy as they
pass through the inner solar system. "If X-rays are observed
from the known location of a comet, one can conclude with great
confidence that the X-rays originated from the comet," Gombosi said.

The University of Michigan team used March 27, 1996, solar
wind density measurements from NASA's WIND spacecraft. Their
model first considers the global interaction of the solar wind
with the comet. It projects the comet into a three-dimensional
grid that automatically applies finer resolution where more
activity occurs. This physics component predicts the
deflective paths and speed of the solar wind traveling through
the comet.

Other co-authors of the "Science" paper are Roman Haberli,
Darren De Zeeuw and Kenneth Powell. The University of Michigan
team is one of nine Grand Challenge Investigations funded by
the NASA High Performance Computing and Communication Program's
Earth and Space Sciences Project. Additional funding comes from
NASA's Office of Space Science, the National Science Foundation
and the Swiss National Science Foundation.